In our previous study of partial sleep deprivation (PSD), we observed that partial sleep deprivation was more effective if subjects were kept awake in the latter part of the night, i.e. after 2 a.m., than if they were kept awake earlier in the night. Patients also slept less on this condition, and the amount of sleep was inversely related to improvement. Thus from our earlier study it is not possible to know whether the timing of sleep is an essential factor in the antidepresssant effects of PSD, but it appears that the amount of sleep reduction, and in particular the reduction in REM sleep, are correlated with clinical response. We also oberved in our previous study that the antidepressant effects of a single night of PSD could be enhanced and extended by a second night of treatment and we have hypothesized that repeating PSD over an extended period might produce antidepressant effects comparable to other somatic therapies. Our present work with partial sleep deprivation consists of two studies. The first is a replication of our earlier project in which we compare the efficacy of sleep deprivation early in the night, i.e. before 3 a.m., with PSD late in the night after 3 a.m. In the present study the duration of sleep on the two treatments is precisely regulated so that unequal sleep durations will not be a source of variance on the different conditions. The second study is a clinical trial of repeated partial sleep deprivation treatments, performed over a three week period. This preliminary study will enable us to assess whether partial sleep deprivation may be useful in the clinical management of depressed patients.